[Hydrogen bonding and electrostatic interactions contribute significantly to the] consolidation of this assembly in the interfacial region, while dispersive forces between the lipid hydrocarbon chains stabilize the core of the membrane.

Even though each membrane exhibits functions unique to that membrane, general functions common to all membranes, can be distinguished. The first basic function of biomembranes is to provide different spatial compartments in living organisms. Compartmentalization (the physical separation of one compartment from another) supplies morphological identity to the cell and its organelles. Biomembranes act as selective barriers for the exchange of molecules between the different compartments, and ultimately, protect the internal microenvironments from the variability and fluctuations of their surroundings. They sustain concentration gradients of chemical species from one side to the other and the cell makes use of the membrane to create, maintain or utilize the energy stored in these concentration gradients. The bilayer matrix provides a two dimensional network in which various functional molecules such as enzymes are specifically distributed and oriented. Lipids act not only as solvent but also as anchors, activators and conformational stabilizers for proteins, which carry out specific catalytic and translocation functions. Another important aspect is the transduction of molecular information across and along membranes. For instance, receptors located on the cell surface receive extracellular signals that are conveyed to the cell interior, which alters it [sic] behavior in response.

The major components of membranes are lipids and proteins. Depending on the type of membrane, their relative amounts vary significantly, ranging from about 20% protein (dry weight) to 80% protein. The most striking feature of membrane lipids is their very large diversity. Considering the possible number of structural and conformational lipid isomers, a hundred of components is involved in eukaryotic cells. In spite of this diversity, only a few classes of lipids predominate in membranes of a given type of organisms.

The glycerophospholipids are the predominant phospholipids found in biological membranes. They are derivatives of glycerol phosphate and contain an asymmetric carbon atom. Two glycerol hydroxyls are linked to hydrophobic hydrocarbon chains.

Hydrogen bonding and electrostatic interactions, however, contribute significantly to the consolidation of this assembly in the interfacial region, while dispersive forces between the lipid hydrocarbon chains stabilize the core of the membrane.

Figure 1.1: Schematic representation of a typical biomembrane.

1.1.1.3 Functions

Even though each membrane exhibits functions unique to that membrane, general functions, common to all membranes, can be distinguished.

The first basic function of biomembranes is to provide different spatial compartments in living organisms. Compartmentalization, i.e. the physical separation of one compartment from another, supplies morphological identity to the cell and its organelles.

Biomembranes act as selective barriers for the exchange of molecules between the different compartments, and ultimately, protect the internal microenvironments from the variability and fluctuations of their surroundings. They sustain concentration gradients of chemical species from one side to the other and the cell makes use of the membrane to create, maintain, or utilize the energy stored in these concentration gradients.

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The bilayer matrix provides a two-dimensional network in which various functional molecules such as enzymes are specifically distributed and oriented. Lipids act not only as solvent but also as anchors, activators, and conformational stabilizers for proteins which carry out specific catalytic and translocation functions.

Another important aspect is the transduction of molecular information across and along membranes. For instance, receptors located on the cell surface receive extracellular signals that are conveyed to the cell interior which alters its behavior in response.

[...]

1.1.2 Composition

The major components of membranes are lipids and proteins. Depending on the type of membrane, their relative amounts vary significantly, ranging from about 20% protein (dry weight) in myelin to 80% protein in mitochondria.

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1.1.2.1 Membrane lipids

The most striking feature of membrane lipids is their very large diversity. Considering the possible number of structural and conformational lipid isomers, a hundred of components is involved in eukaryotic cells. In spite of this diversity, only a few classes of lipids predominate in membranes of a given type of organisms. In this section, the main classes of lipids are described with regard to their molecular characteristics.

Glycerophospholipids The glycerophospholipids are the predominant phospholipids found in biological membranes. They are derivatives of glycerol phosphate and contain an asymmetric carbon atom. Two glycerol hydroxyls are linked to hydrophobic hydrocarbon chains.